PREFACE
This is the third part of my 'history of a language' series, in which I am going to concentrate on the German language. As a speaker of German and a person interested in Germanic linguistics, I found the German language interesting for various reasons that I am going to discuss now.

CLASSIFICATION
The German language, also known as High German (Hochdeutsch), belongs to the Germanic branch of the Indo-European language family and is thus relative to Celtic, Balto-Slavic, Armenian, Paleo-Balkan and other languages of the same family. It is not to be confused with Low German language, which is known as Low Saxon and is descendant of Old Saxon. German itself is descendant of Old High German.

GERMANIC PROTO-LANGUAGE
The first Germanic language, Proto-Germanic, dates at the latter half of the 1st millenniun B.C. and it is generally agreed that its urheimat was in the present-day Scandinavia, particularly southern Sweden and Denmark. The ancestors of Germanic peoples settled in the area some time around 2500 B.C. The differentiation of Proto-Germanic from the other Indo-European dialects/languages began in the 6th century, by so-called Grimm's law, also known as the Germanic consonantal shift. I am not going to discuss it in detail here since this essay is about German language.

FROM COMMON GERMANIC TO OLD HIGH GERMAN
The first migration of Germanic tribes out of their urheimat took place in the 8th century B.C., but in greater number occurred first in the latter half of the 1st millennium B.C. At this time the Common Germanic language dissolved into three separate groups - the western/Ingvaeonic group, the northern/Istvaeonic group (speakers of the Proto-Norse language) and the eastern/Irminonic group (ancestoral to Gothic, Burgundic, Longobardic, Vandalic and others). In the first half of the 1st millenniun, the Ingvaeonic group dissolved into several languages, most notably Old Frankish, Old Saxon, Old Frisian and Old High German, which I will discuss in detail.

FROM INGVAEONIC TO OLD HIGH GERMAN
Old High German is a language with relatively low non-Germanic influence (unlike Old English or Old Low Franconian). Together with Old Frankish (later Old Low Franconian), it was the language spoken in the Frankish Empire, but at the time, Franks didn't distinguish between them, they considered themselves to speak roughly the same language called 'the language of the people' - diutisch (whence Deutsch, 'diut' meant folk, people) and 'the language of the Franks' - frankisch (whence Français). It was only when the Old Frankish/Franconian was replaced by Old French that 'diutisch' and 'frankisch' got different meanings.

The grammar was quite similar to the modern one, so it is not worth the discussion here. What is important for German is the High German consonantal shift, which started in 4th century and ended around 10th century. I am going to provide a longer list here:

Shift - Proto-Gmc - Old HG - German - English/Swedish/Dutch
p→f - skipan - skif - Schiff - ship/skepp/schip
p→pf - aplaz - apful - Apfel - apple/äpple/appel
t→zz - etanan - ezzan - essen - eat/äta/eten
t→ts - tidiz - zít - Zeit - tide/tid/tijd
k→hh - makó (!) - mahhó - machen - make/???/maken
d→t - dagaz - tag - Tag - day/dag/dag
v→b - khaf - haben - haben - have/ha/haven
s→ʃ - spenwanan - spinnan - spinnen - spin//spinnen

Quite a common shift in later Germanic languages (only English and Icelandic lack it):
þ/ð→d - þurh - duruh - durch - through/???/door

(!) - the form is Ingvaeonic, it seems that it didn't appear in the other two branches, therefore there is no Swedish cognate

The Old Norse form of Swedish 'ha' was 'hafa'.

THROUGH MIDDLE AND EARLY MODERN HIGH GERMAN TO STANDARDISATION
Middle High German was spoken from 11th to 14th century, when the language changed and simplified (lost a number of inflexion) and enterred the stage of Early Modern High German, which was almost identical to the present-day form. For comparison, I am providing the first few verses of John's Gospel from Martin Luther's translation:

Early Modern High German:
Im anfang war das Wort, vnd das Wort war bey Gott, vnd Gott war das Wort,
das selbige war im anfang bey Gott.
Alle ding sind durch dasselbige gemacht, vnd on das selbige ist nichts gemacht, was gemacht ist.

Current form:
Im Anfang war das Wort, und das Wort war bei Gott, und Gott war das Wort.
Dasselbe war im Anfang bei Gott.
Alle Dinge sind durch dasselbe gemacht, und ohne dasselbe ist nichts gemacht, was gemacht ist.

As you can see, there are very little differences and the speaker of German will sure understand Luther's version as well.

The standardisation process was successfully completed by mid 18th century and the first dictionary of the Brothers Grimm, the 16 parts whereof were issued between 1852 and 1860, remains the most comprehensive guide to the words of the German language. Brothers Grimm remain two of the most important figures in both German and Germanic linguistics (Grimm's law is named after one of them, Jacob Grimm, who discovered it).